In the current state of known embodiments, magnetography is essentially a digital technique, which makes it possible to have grey scales but to the detriment of the image resolution. Hence for a given addressability, the dots making up the image are either magnetically unsaturated or as close as possible to saturation. As a result, the reflectance of the positions in the mapping grid assume only two values, white and black. Under these conditions, a scale of grey shades can be made up only by using a picture element or "pixel" composed of several dots, by a method sometimes called "dithering". In general, the pixel is made up of a square n.times.n in size, thus offering n.sup.2 different levels of grey, as shown in FIG. 5. Digital resolution of the greys by using a pixel with n.times.n dots has the grave disadvantage of reducing the image resolution R in the ratio n. For example, for an addressability that represents the spatial density of the theoretical grid of the addressable dots, that is, 240 dots per inch (dpi) that can be written upon and a 4.times.4 pixel allowing 16 grey levels, The image resolution drops to 60 pixels per inch (ppi), which is manifestly too low for the eye to correctly integrate the reflectances of the various dots of the pixel so as to obtain the mean value thereof. If one wishes to preserve an image resolution of 240 ppi, then the alternative consists of using addressability that is four times greater, that is, 960 dpi. Obtaining such densities cannot be done without encountering serious problems in making the requisite writing head bars in high-speed parallel printers.